The presently disclosed embodiments relate in general to electrophotographic imaging members which are provided with a novel charge transport layer. In particular, the imaging members of present embodiments are negatively charged members comprising an improved charge blocking layer designed with a specific material composition having great capacity to prevent and/or stop hole injections into the imaging layer(s) from the conductive ground plane during the electrophotographic imaging process. The prepared imaging member, having the improved charge blocking layer, has stabilized cyclic photo-electrical properties to impact copy printout quality enhancement and extends service life when used in an electrophotographic imaging system. The present embodiments also provide a process for making and using these imaging members that further meet the service life function objectives in the field.
In electrophotographic reproducing apparatuses, including digital, image on image, and contact electrostatic printing apparatuses, a light image of an original to be copied is typically recorded in the form of an electrostatic latent image upon a photosensitive member and the latent image is subsequently rendered visible by the application of electroscopic thermoplastic resin particles and pigment particles, or toner. Electrophotographic imaging members are well known in the art. Typically, the electrophotographic imaging members are, for example, photoreceptors) commonly utilized in electrophotographic (xerographic) processing system. Generally, these imaging members comprise at least a supporting substrate and at least one imaging layer comprising a thermoplastic polymeric matrix material. In an electrophotographic imaging member or photoreceptor, the photoconductive imaging layer may comprise only a single photoconductive layer or multiple of layers such as a combination of a charge generating layer and one or more charge transport layer(s).
Electrophotographic imaging members can have two distinctively different configurations. For example, they can comprise a flexible member, such as a flexible scroll or a belt containing a flexible substrate. Since typical flexible electrophotographic imaging members exhibit spontaneous upward imaging member curling after completion of solution coating the outermost exposed imaging layer, an anticurl back coating is therefore required to be applied to back side of the flexible substrate support to counteract/balance the curl and provide the desirable imaging member flatness. Alternatively, the electrophotographic imaging members can also be a rigid member, such as those utilizing a rigid substrate support drum. For these drum imaging members, having a thick rigid cylindrical supporting substrate bearing the imaging layer(s), there is no exhibition of the curl-up problem, and thus, there is no need for an anticurl back coating layer.
Although the scope of the present disclosure covers the preparation of all types of electrophotographic imaging members in either a rigid drum design or a flexible belt configuration, but for reasons of simplicity, the embodiments and discussion following hereinafter will be focused solely on and represented by electrophotographic imaging members in the flexible belt configuration.
Electrophotographic flexible belt imaging members may include a photoconductive layer including a single layer or composite layers. The flexible belt electrophotographic imaging members may be seamless or seamed belts. Seamed belts are usually formed by cutting a rectangular sheet from a web, overlapping opposite ends, and welding the overlapped ends together to form a welded seam. Typical electrophotographic imaging member belts include a charge transport layer and a charge generating layer on one side of a supporting substrate layer and an anticurl back coating coated onto the opposite side of the substrate layer. By comparison, a typical electrographic imaging member belt does, however, have a more simple material structure; it includes a dielectric imaging layer on one side of a supporting substrate and an anti-curl back coating on the opposite side of the substrate to render flatness. Since typical negatively-charged flexible electrophotographic imaging members exhibit undesirable upward imaging member curling after completion of coating the top outermost charge transport layer, an anticurl back coating, applied to the backside, is required to balance the curl. Thus, the application of anticurl back coating is necessary to provide the appropriate imaging member with desirable flatness.
One type of composite photoconductive layer used in xerography is illustrated in U.S. Pat. No. 4,265,990 which describes a negatively-charged photosensitive member having at least two electrically operative layers. One layer comprises a photoconductive layer which is capable of photogenerating holes and injecting the photogenerated holes into a contiguous charge transport layer.
Photosensitive members having at least two electrically operative layers, as disclosed above, provide excellent electrostatic latent images when charged in the dark with a uniform negative electrostatic charge, exposed to a light image and thereafter developed with finely divided electroscopic marking particles. The resulting toner image is usually transferred to a suitable receiving member such as paper or to an intermediate transfer member which thereafter transfers the image to a receiving member such as paper.
In the case where the charge generating layer (CGL) is sandwiched between the outermost exposed charge transport layer (CTL) and the electrically conducting layer, the outer surface of the charge transport layer is charged negatively and the conductive layer is charged positively. The CGL then should be capable of generating electron hole pair when exposed image wise and inject only the holes through the charge transport layer. In the alternate case when the charge transport layer is sandwiched between the CGL and the conductive layer, the outer surface of Gen layer is charged positively while conductive layer is charged negatively and the holes are injected through from the CGL to the charge transport layer. The CTL should be able to transport the holes with as little trapping of charge as possible. In a typical flexible imaging member web like photoreceptor, the charge conductive layer may be a thin coating of metal on a flexible substrate support layer.
As more advanced, higher speed electrophotographic copiers, duplicators and printers were developed, however, degradation of image quality was encountered during extended cycling. The complex, highly sophisticated duplicating and printing systems operating at very high speeds have placed stringent requirements including narrow operating limits on photoreceptors. For example, the numerous layers used in many modern photoconductive imaging members must be highly flexible, adhere well to adjacent layers, and exhibit predictable electrical characteristics within narrow operating limits to provide excellent toner images over many thousands of cycles. One type of multilayered photoreceptor that has been employed as a belt in electrophotographic imaging systems comprises a substrate, a conductive layer, an optional blocking layer, an optional adhesive layer, a charge generating layer, a CTL and a conductive ground strip layer adjacent to one edge of the imaging layers, and an optional overcoat layer adjacent to another edge of the imaging layers. Such a photoreceptor usually further comprises an anticurl back coating layer on the side of the substrate opposite the side carrying the conductive layer, support layer, blocking layer, adhesive layer, CGL, CTL and other layers.
Typical negatively-charged imaging member belts, such as flexible photoreceptor belt designs, are made of multiple layers comprising a flexible supporting substrate, a conductive ground plane, a charge blocking layer, an optional adhesive layer, a CGL, and a CTL. The CTL is usually the last layer to be coated to become the outermost exposed layer and is applied by solution coating then followed by drying the wet applied coating at elevated temperatures of about 115° C., and finally cooling it down to ambient room temperature of about 25° C. When a production web stock of several thousand feet of coated multilayered photoreceptor material is obtained after finishing the CTL coating through drying/cooling process, upward curling of the multilayered photoreceptor is observed.
This upward curling is a consequence of thermal contraction mismatch between the CTL and the substrate support. Since the CTL in a typical photoreceptor device has a coefficient of thermal contraction approximately 3.7 times greater than that of the flexible substrate support, the CTL exhibits a larger dimensional shrinkage than that of the substrate support as the imaging member web stock (after through elevated temperature heating/drying process) as it cools down to ambient room temperature. This dimensional contraction mis-match results in tension strain built-up in the CTL, at this instant, is pulling the imaging member web stock upward to exhibit curling. If unrestrained at this point, the imaging member web stock (for example, one comprising a 24 micrometer polycarbonate-diamine imaging layer and a 3.5 mil polyethylene terephthalate substrate) will spontaneously curl upwardly into a 1.5-inch roll. To offset the curling, an anticurl back coating is applied to the backside of the flexible substrate support, opposite to the side having the charge transport layer, and render the imaging member web stock with desired flatness.
Conventional photoreceptors are disclosed in the following patents, a number of which describe the presence of light scattering particles in the undercoat layers: U.S. Pat. No. 5,660,961; U.S. Pat. No. 5,215,839; and U.S. Pat. No. 5,958,638. The term “photoreceptor” or “photoconductor” is generally used interchangeably with the terms “electrophotographic imaging member.” The terms “charge transport molecule” are generally used interchangeably with the terms “hole transport molecule.” And also, the term “charge blocking layer” is generally used interchangeably with the term “hole blocking layer”.
Relevant prior arts to the present disclosure are collectively summarized for reference and presented in the following.
In U.S. Pat. No. 7,544,452, it discloses an electrophotographic imaging member comprising a thick undercoat layer further comprising specific binders. The binders contain metal oxide nanoparticles and a co-resin of phenolic resin and aminoplast resin.
In U.S. Pat. No. 4,584,253, it discloses an electrophotographic imaging member comprising a charge generation layer, a contiguous charge transport layer comprising an aromatic amine or hydrazone charge transport molecules in a continuous polymeric binder phase, and a hydroxyl alkyl cellulosic hole trapping material located on the same side of the charge transport layer as the charge generation layer. The hydroxyl alkyl cellulosic hole trapping material being used is free of electron withdrawing groups. A process for using this electrophotographic imaging member is also disclosed.
In U.S. Pat. No. 5,008,169, it discloses an electrophotographic imaging member comprised of a supporting substrate, a ground strip layer, a hole blocking-adhesive layer comprised of a polyphosphazene, including polyorganophosphazeness, a photogenerating layer, and a hole transport layer.
In U.S. Pat. No. 5,378,566, it discloses a structurally simplified electrophotographic imaging member including a substrate, a hole blocking/adhesive layer, a charge generation layer, and a charge transport layer, the hole blocking/adhesive layer including a film forming binder having dispersed therein a particulate reaction product of metal oxide particles and a hydrolyzed reactant selected from the group consisting of a nitrogen containing organo silane, an organotitanate and an organozirconate and mixtures thereof. In embodiments, the imaging member is free of any distinctive adhesive layer in contiguous contact with the hole blocking/adhesive layer. This imaging member may be utilized in an electrophotographic imaging process
In U.S. Pat. No. 5,660,961, it discloses an electrophotographic imaging member including a substrate, a charge blocking layer, an optional adhesive layer, a charge generating layer, and charge transport layer, the blocking layer comprising solid finely divided light scattering inorganic particles having an average particle size of from about 0.3 micrometer to about 0.7 micrometer selected from the group consisting of amorphous silica, mineral particles an mixtures thereof, dispersed in a matrix material comprising the chemical reaction product of (a) a film-forming polymer selected from the group consisting of hydroxyl alkyl cellulose, hydroxyl alkyl methacrylate polymer, hydroxyl alkyl methacrylate copolymer, and mixtures thereof and (b) an organosilane.
Thus, electrophotographic imaging members (comprising a supporting substrate, having a conductive surface on one side, directly coated over a charge blocking layer with subsequent photo-electrically active layers and an anticurl back coating layer coated on the other side of the supporting substrate) used in the negative charging system do still exhibit deficiencies which are undesirable in advanced automatic, cyclic electrophotographic imaging copiers, duplicators, and printers. While the above mentioned electrophotographic imaging members may be suitable or limited for their intended purposes, further improvement on these imaging members are needed. For example, there continues to be a need for improvements in such systems, particularly for an imaging member belt that includes an active charge blocking layer that is easy to apply by a solution coating process and with effective hole blocking property to enhance image printout quality free of spot defects in the output copies.